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Chapter 11:
LATERAL DRIFT AND VERTICAL DEFLECTIONS

By Alexander Newman

From Metal Building Systems: Design and Specifications, Second Edition

11.1 THE MAIN ISSUES

The discussion in Chaps. 7 and 9 has already highlighted an importance of specifying correct design criteria for lateral drift and vertical deflections. This chapter is specifically devoted to this critical and controversial topic that occupies the minds of many structural engineers who specify metal building systems.

First, the definitions: Lateral (story) drift is the amount of sidesway between two adjacent stories of a building caused by lateral (wind and seismic) loads (Fig. 11.1). For a single-story building, lateral drift equals the amount of horizontal roof displacement. Horizontal deflection of a wall refers to its horizontal movement between supports under wind or earthquake loading. Vertical deflection of a floor or roof structural member is the amount of sag under gravity or other vertical loading.

FIGURE 11.1: Story drift caused by lateral loads.

Why is any of this controversial? Drift and deflection criteria, along with some other issues such as vibrations, deal with serviceability, or functional performance, of buildings under load. Model building codes have traditionally prescribed the desired levels of strength and safety, leaving the more nebulous topics of satisfying the occupants perceptions of comfort and solidity up to the designers. The designers criteria for achieving these goals are necessarily subjective, as the building which seems flimsy to one person may feel comfortable to another.

Various design firms tend to specify similar, although not identical, limits on horizontal and vertical building displacements for medium and high-rise structures. On the other hand,...

Spring washers, sometimes called disc springs, lend their mechanical capabilities to the unique profile of the material: the irregularities of the washer compress with a proportionate resistance to return to their predeflected shape. Spring washers are employed in applications where assemblies need a part to take up play, maintain assembly tension, compensate for expansion or contraction in materials, or to absorb intermittent shock loads and provide a controlled reaction under dynamic loads.

Curtain walls are lightweight exterior cladding systems that attach to a building or structure, usually from floor to floor, to provide a façade. They modify the exterior appearance, but do not carry the dead load of the building or structure.

Topics of Interest

12.1 INTRODUCTION
Foundations, contrary to the dreams of building owners, do not come prepackaged with metal building systems. The concept of single-source responsibility for pre-engineered buildings...

13.2 Curtain-Wall Problems Caused by Structural Forces and Movements 13.2.1 Problems Caused by Lateral Loads
Curtain walls do play a structural role and a critical one at that in resisting wind loads...